Those cards are generally for support of Malcolm X.
For those who do not know who I am talking about. Malcolm X - Wikipedia, the free encyclopedia
They are also used in Jamaican Flag placards, etc.
I am sure that it does not mean SEX lol. The term X was used just because it was frowned upon to say sex in Pakistan.
I remember in high school it would be more appropriate to say x instead of sex.
IE, Yaar aaj tunai x macha dia instead of yaar aaj tunai sex macha dia...
Case and point.